Inductive coupling has been used for many years to enable users of hearing aids to listen to public address systems or to use telephone receivers clearly and with the exclusion of interfering sounds. Inductive coupling works by setting up a magnetic field, the intensity of which fluctuates with time, in proportion to the audio signal. The magnetic field is converted to an electrical signal within the hearing aid and then amplified to produce an audible output.
Cordless telephones have become popular in recent years and look set to become even more popular in the future. There are, however, certain technical problems which make it difficult to adapt cordless telephones for use with hearing aids by inductive coupling.
One particular class of cordless telephones, shortly to be launched in the UK and going under the code name of "CT2" uses a system called "burst mode duplex". The same radio channel is used for both transmission and reception, with the two radios at the ends of the communication link alternately transmitting a burst of information. The process of burst mode duplexing (sometimes called "ping-pong" or "time division duplex") involves switching the radio transmitter within the handset on and off alternately as the handset first transmits and then receives. In the case of CT2 products, the bursts occur at a rate of 500 per second and the internal current consumption of the handset therefore fluctuates at a rate of 500 Hz.
In attempting to make such a telephone handset compatible with hearing aids it is found that there is a stray magnetic field associated with the fluctuation in power supply current. The sensitivity of the hearing aid and the magnitude of the fluctuation are such that there is substantial interference at the duplexing burst rate. The present invention aims to provide a burst mode transmitter/receiver unit in which interference from this source is suppressed.